In order to decrease contamination and to enhance throughput, semiconductor processing systems often utilize one or more robots to transfer semiconductor wafers, substrates and other workpieces between a number of different vacuum chambers which perform a variety of tasks. An article entitled “Dry Etching Systems: Gearing Up for Larger Wafers”, in the October, 1985 issue of Semiconductor International magazine, pages 48–60, describes a four-chamber dry etching system in which a robot housed in a pentagonal-shaped mainframe serves four plasma etching chambers and a loadlock chamber mounted on the robot housing. In order to increase throughput, it has been proposed to utilize two loadlock chambers as described in U.S. Pat. No. 5,186,718. In such a two loadlock system, both loadlock chambers are loaded with full cassettes of unprocessed wafers. FIG. 1 of the present application illustrates two typical loadlock chambers LLA and LLB, each having a cassette 190 therein for holding unprocessed wafers 192 to be unloaded by a robot 194 and transferred to various processing chambers 196 attached to a mainframe 198.
The loadlock chamber LLA, for example, is a pressure-tight enclosure which is coupled to the periphery of the mainframe 198 by interlocking seals which permit the loadlock chamber to be removed and reattached to the mainframe as needed. The cassette 190 is loaded into the loadlock chamber LLA through a rear door which is closed in a pressure-tight seal. The wafers are transferred between the mainframe 198 and the loadlock chamber LLA through a passageway 199 which may be closed by a slit valve to isolate the loadlock chamber volume from the mainframe volume.
As shown in FIG. 2, a typical cassette 190 is supported by a platform 200 of a cassette handler system 208 which includes an elevator 210 which elevates the platform 200 and the cassette 190. The platform 200 has a top surface which defines a base plane 220 on which the cassette 190 rests. As the cassette includes a plurality of “slots” 204 or wafer support locations, the elevator moves the cassette to sequentially position each of the slots with the slit valves to allow a robot blade to pass from the mainframe, through the slit valve, and to a location to “pick” or deposit a wafer in a wafer slot.
The slots 204 of the cassette may be initially loaded with as many as 25 or more unprocessed wafers or other workpieces before the cassette is loaded into the loadlock chamber LLA. After the loadlock access door is closed and sealed, the loadlock chamber is then pumped by a pump system down to the vacuum level of the mainframe 198 before the slit valve is opened. The robot 194 which is mounted in the mainframe 198 then unloads the wafers from the cassette one at a time, transferring each wafer in turn to the first processing chamber. The robot 194 includes a robot hand or blade 206 which is moved underneath the wafer to be unloaded. The robot 194 then “lifts” the wafer from the wafer slot supports supporting the wafers in the cassette 190. By “lifting,” it is meant that either the robot blade 206 is elevated or the cassette 190 is lowered by the handler mechanism 208 such that the wafer is lifted off the cassette wafer supports. The wafer may then be withdrawn from the cassette 190 through the passageway and transferred to the first processing chamber.
Once a wafer has completed its processing in the first processing chamber, that wafer is transferred to the next processing chamber (or back to a cassette) and the robot 194 unloads another wafer from the cassette 190 and transfers it to the first processing chamber. When a wafer has completed all the processing steps of the wafer processing system, and two cassettes full of wafers are loaded in the loadlocks, the robot 194 returns the processed wafer back to the cassette 190 from which it came. Once all the wafers have been processed and returned to the cassette 190, the cassette in the loadlock chamber is removed and another full cassette of unprocessed wafers is reloaded. Alternatively, a loaded cassette may be placed in one loadlock, and an empty one in the other loadlock. Wafers are thus moved from the full cassette, processed, and then loaded into the (initially) empty cassette in the other loadlock. Once the initially empty cassette is full, the initially full cassette will be empty. The full “processed” cassette is exchanged for a full cassette of unprocessed wafers, and these are then picked from the cassette, processed, and returned to the other cassette. The movements of the robot 194 and the cassette handler 208 are controlled by an operator system controller 222 (FIG. 1) which is often implemented with a programmed workstation.
As shown in FIGS. 2 and 3, the wafers are typically very closely spaced in many wafer cassettes. For example, the spacing between the upper surface of a wafer carried on a moving blade and the lower surface of an adjacent wafer in the cassette may be as small as 0.050 inches. Thus, the wafer blade must be very thin, to fit between wafers as cassettes are loaded or unloaded. As a consequence, it is often important in many processing systems for the cassette and the cassette handler 208 to be precisely aligned with respect to the robot blade and wafer to avoid accidental contact between either the robot blade or the wafer carried by the blade and the walls of the cassette or with other wafers held within the cassette.
However, typical prior methods for aligning the handler and cassette to the robot blade have generally been relatively imprecise, often relying upon subjective visual inspections of the clearances between the various surfaces. Some tools have been developed to assist the operator in making the necessary alignments. These tools have included special wafers, bars or reference “pucks” which are placed upon the robot blade and are then carefully moved into special slotted or pocketed receptacles which are positioned to represent the tolerance limits for the blade motions. However, many of these tools have a number of drawbacks. For example, some tools rely upon contact between the blade or a tool on the blade and the receptacle to indicate a condition of nonalignment. Such contact can be very detrimental to high precision mechanisms for moving the blade as well as the blade itself. Also, many such tools do not indicate the degree of alignment or nonalignment but merely a “go/no-go” indication of whether contact is likely.
In aligning the handler mechanism to the robot blade, one procedure attempts to orient the cassette to be as level as possible with respect to the robot blade. One tool that has been developed to assist in the leveling procedure has dual bubble levels in which one bubble level is placed on the blade and the other is placed on the cassette. The operator then attempts to match the level orientation of the blade to that of the cassette. In addition to being very subjective, such bubble tools have also often been difficult to see in the close confines of the cassette and handler mechanisms.
As a consequence of these and other deficiencies of the prior alignment procedures and tools, alignments have often tended to be not only imprecise but also inconsistent from application to application. These problems have frequently lead to the breakage or scratching of very expensive wafers and equipment as well as the generation of damaging particulates in the systems.